1. Field of the Invention
The present invention is directed to digital cinema projectors, and more particularly to a high performance compound reflector and cooling system for use with projection lamps such as Xenon (Xe) lamps.
2. Description of the Related Art
High-end projectors that are capable of 8000 lumens and higher, typically use Xe bubble lamps coupled to an ellipsoidal reflector. The reflected light is captured from the first focal point of the reflector and is re-imaged at a second focal point. The second focal point is usually co-incident with an integrator rod or some other means of homogenizing the light. Illumination optics is then used to image the light from the integrator rod onto a light valves for projection. Typically, the ellipsoidal reflector contains a secondary spherical reflector. This allows the ellipsoidal reflector to be more compact, thereby reducing the size of the projector without sacrificing light collection efficiency of the reflector.
Reflectors used for lamps in the 1–6 KW range are typically made of metal (e.g. nickel deposit) or glass. Along with reflecting visible light, the reflectors also serve the purpose of removing infrared light generated by the lamp from the second focal point. This helps in reducing the amount of heating within the projector and is vital for projector performance. With metal reflectors this is accomplished by using IR absorbing coatings at the reflector surface. IR transmitting coatings tend to be used in glass reflectors. In either case, effective cooling must be applied to the reflector and the lamp for proper operation.
To that end, air is usually directed onto the reflector surface, either directly or by the use of ducting, to maintain the reflector temperature and temperature gradient below a predetermined threshold above which damage can occur. The most straightforward method is to force air axially onto the reflector. However, this is not usually done because of size constraints. In most cases air is forced onto the side of the reflector and ducting is used to re-distribute the air. This often results in areas of the reflector having high temperature gradients, resulting in local distortion and reduced coupling efficiency. Typically, for metal and glass reflectors operating with 0 KW lamps, up to 800 cfm of air flow is required for effective cooling. The lamp ends and bulb also need to be cooled. When air is used to cool the reflector from the side, effective cooling occurs for one lamp end. To cool the hub, air must be forced through the back opening of the reflector, over a first end of the bulb, over the center of the bulb and finally over the opposite end. However, air flow over the opposite end is usually too low for cooling. A second fan and/or complex extra ducting from the primary fan is therefore often used to provide cooling for the opposite lamp end.
In order to make smaller and brighter projectors the reflector size must be decreased while the lamp power remains the same. The smallest reflectors for a given lamp power are made of glass. This is because IR transmissive coatings can be used to remove the lamp heat. In the use of such coatings, the glass temperature and therefore coating temperature remain lower than if the reflector were made of metal. Thus a smaller reflector can be made.
Nonetheless, ensuring cost effective and efficient cooling of the reflector for high power Xe lamps (i.e. greater than 1 KW) while optimizing light collection on the integrator rod, remains a difficult challenge to projector designers.